1. Field of the Invention
This invention relates to a laser light beam generating apparatus. More particularly, the present of invention relates to a laser light beam generating apparatus in which a laser light beam converted wavelength is generated by a non-linear optical crystal element.
2. Background of the Invention
It has hitherto been proposed to realize wavelength conversion by taking advantage of the high power density within a resonator. For example, second harmonic generation (SHG) is often achieved by placing a non-linear optical crystal in an external resonator in an attempt to improve the efficiency of the wavelength conversion.
As an SHG used the non-linear optical crystal element providing the resonator, the resonator which includes at least a pair of mirrors, a laser medium and a non-linear optical crystal element is known. In this resonator, the laser medium and the non-linear optical crystal element are provided between the pair of mirrors. With this type of the laser light beam generating apparatus, the second harmonic laser light beam is taken out efficiently by phase matching the second harmonic laser light beam with respect to the laser light beam of the fundamental wavelength by a non-linear optical crystal element provided within the resonator.
There is also known an external resonant method according to which a laser light beam from a laser light source is introduced into an external resonator as laser light beam of a fundamental wavelength and propagated through a non-linear optical crystal element back and forth for a resonant operation to generate a second harmonic laser light beam. In the external resonant method, the finesse value of the resonator, corresponding to a Q-value of resonation, is set to a larger value of about 100 to 1000 to set the light density within the resonator to a value hundreds of times as large as the incident light density. As a result, this type resonator can take advantage effectively of non-linear effects of the non-linear optical crystal element within the resonator.
Meanwhile, for producing laser light beams of second or higher harmonics or so-called sum frequency according to the external resonant method, it is necessary to realize extremely fine position control of limiting changes or errors of the optical path length of the resonator to less than 1/1000 or 1/10000 of the wavelength, that is less than 1 .ANG..
The conventional practice in limiting the resonator length has been to have the reflective mirrors of the resonator supported by stacked piezoelectric elements by so-called PZT and to feed an error signal proportional to changes in the resonator length back to the stacked piezoelectric elements to complete a servo loop for automatically controlling and stabilizing the resonator length.
In general, piezoelectric elements have multiple resonance at intervals of several to tens of kilohertz frequencies and have phase delay over the entire frequency range due to self capacitance. As a result, it is .difficult to spread frequency range of the servo range to several kilohertz. Since the stacked piezoelectric elements are in need of a high driving voltage of hundreds to thousands of volts the driving electric circuit is complicated and expensive.